The number of connecting terminals which are used in an automobile, a mobile phone and the like tends to be increased in accordance with increase of the number of electronic control devices to be used therein. It has been required for the connecting terminal to be miniaturized and lightened from the viewpoint of improvement in fuel efficiency of an automobile, space saving, portability of a mobile phone and the like. In order to respond to these requirements, it is necessary that the connecting terminal is prevented from deformation due to force (insertion force) which is applied when the connecting terminal is fitted to another connecting terminal, and that contact pressure between the connecting terminals at their connected portion is maintained. Accordingly, it has been required for a material which has hitherto been used in connecting terminals to use a material having a strength higher than conventional copper alloys. In addition, it has been required for a material used in a connecting terminal which is used under high temperature environment such as an engine room of an automobile to use a material which is excellent in stress relaxation resistance in order to suppress lowering of contact pressure between the connecting terminals at their connected portion due to heating with the passage of time.
In recent years, it has been investigated to develop a copper alloy by adding various metals to a copper alloy in order to increase mechanical strength of a connecting terminal, and improve stress relaxation resistance of the connecting terminal. However, a copper alloy which can be applied to a miniaturized connecting terminal has not yet been developed at the present time.
On the other hand, a stainless steel plate is suitable from the viewpoint of miniaturization, lightening and reduction in cost, since the stainless steel plate has mechanical strength higher than a copper alloy, and is excellent in stress relaxation resistance, small in specific gravity and inexpensive. As an electrical contact member formed of a stainless steel plate, there has been proposed an electrical contact member made of a stainless steel, in which a Ni plating layer is formed on a stainless steel plate which is used as a base material, and an Au plating layer is partially formed on the Ni plating layer (see, for example, Patent Literature 1). According to the electrical contact member, however, the Au plating layer is abraded by repeated fine sliding at the contact portion of a connecting terminal of the electrical contact member, and the stainless steel which is used as a base material is exposed to the outside surface. Therefore, when the stainless steel is oxidized, there is a possibility that a contact resistance between the connecting terminals is increased at the contact portion.
As an electric conductive material for a connecting member, which has low coefficient of friction, and which can maintain reliability of electrical connection, there has been proposed an electric conductive material for a connecting member, in which a Ni coating layer having an average thickness of 3.0 μm or less, a Cu—Sn alloy coating layer having a mean thickness of 0.2 to 3.0 μm and a Sn coating layer are formed on the surface of a Cu plate which is used as a base material in this order; a diameter of a maximum inscribed circle of the Sn coating layer is 0.2 μm or less in a cross section perpendicular to the surface of the above-mentioned material; a diameter of a minimum inscribed circle of the Sn coating layer is 1.2 to 20 μm in a cross section perpendicular to the surface of the above-mentioned material; and the highest difference between the outermost point of the material and the outermost point of the Cu—Sn alloy coating layer is 0.2 μm or less (see, for example, Patent Literature 2). In addition, as an electric conductive material for a connecting member, which corresponds to miniaturization of a terminal, and which is low in insertion force and excellent in electrical reliability, there has been proposed a copper plate for a connecting member, in which a Cu—Sn alloy coating layer, and a Sn or Sn-alloy coating layer is formed on the outermost surface of the copper plate; an arithmetic average roughness Ra is 0.5 μm or more and 4.0 μm or less in a direction parallel to a sliding direction at connection; a mean distance RSm between a valley depth and a peak height of the copper plate is 0.01 mm or more and 0.3 mm or less in the direction as mentioned above; a skewness Rsk is less than 0; and a peak height of the convex portion Rpk is 1 μm or less (see, for example, Patent Literature 3). However, there is a possibility in the above-mentioned electric conductive material for a connecting member and the above-mentioned copper plate for a connecting member that contact resistance increases at the connected portion when sliding between connecting members is repeated.
In recent years, therefore, there has been desired to develop a material for a connecting member which is small in coefficient of friction, and which can suppress increase in contact resistance even when fine sliding of the connecting member is repeated.